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Curing UHPC
Q: How were the Ultra High Performance Concrete
Beams for the Jakway Park Bridge cured?
A: The beams in the Jakway Park Bridge
were made with a particular type of Ultra High Performance Concrete
(UHPC) known as "Ductal®". (This is also sometimes
known as reactive powder concrete). These beams were cured in two
stages.
The first stage involved curing at ambient temperatures, although
steam curing up to 115oF (46.1oC) would be allowed much like that
for most precast, prestressed beams. The PI-girders were covered
with insulating tarps and kept at ambient temperature until match-cast
cylinders indicated a compressive strength of 5.1 ksi (35 MPa) Then
the forms were released and curing at ambient temperatures continued
until the compressive strength of match-cast cylinders attained
14.5 ksi (100 MPa ). At that point, the forms were removed and the
strands were detensioned.
The second stage of curing then began with thermal treatments
applied to the UHPC beams with moisture present. The goal was to
have thermal treatments of approximately 190oF (87.8oC) applied
along with greater than 95% relative humidity for at least 48 hours.
The thermal treatments have been shown to enhance not only the strength
of the member but the durability as well. The beams were wrapped
with insulating tarps and steam was released underneath the girders.
The temperature was increased gradually over a period of approximately
six hours. Once the second curing period was completed, the curing
temperature was decreased gradually over a period of approximately
six hours.
For more information on the curing and material properties of this
type of UHPC, see the Federal Highway Administration Report “Material
Property Characterization of Ultra-High Performance Concrete”,
Report
Number FHWA-HRT-06-103 and Michigan Technological University’s
Report for Michigan DOT “Ultra-High Performance Concrete for
Michigan Bridges, Material Performance-Phase 1”, Report
Number MDOT RC-1525.
More on reactive powder concrete.
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